What is a color ramp?
A Color Ramp is used for many different things. You will use it all the time. 

Here are a few examples. 

It can be used to mix many different colors. 



It can be used to control how much of a texture you see ie Roughness (and other inputs by clamping the white and blacks). 




It can also be used as a mask to mix two materials together. By clamping down how much falloff or fade you want your texture/mask to have. 

There are more use cases but here are some to get you started.